I want to teach you the inner-workings of non-active verbs and negation. In English, infinitives are un-conjugated verbs preceded with "to", like "to teach"; gerunds are verbs being nouns, often with the special suffix "-ing", like "working" or "-ion", like "communication"; and negation is words like "no, not, un-, and non-". Infinitives and gerunds are non-active verbs, and there always must be another active verb in the sentence.

Infinitives

Infinitives are pretty much useless in Sindarin. The only time you will see them is when Tolkien was listing words. But, they exist and you should know how they are made.

A-verb Infinitives are very easy. They are exactly the same as the A-verb Imperatives.
Example: From the root "fence" (thora-) we get "to fence" (thoro).

I-verb Infinitives are very easy as well. They are I-verbs in their Present tense conjugation, without any pronoun suffix added.
Example: From the root "halt" (dar-) we get "to halt" (deri).

Gerunds

A-verb gerunds are very easy to make. Add a (-d) to the root.
Example: From the root "fence" (thora-) we get "fencing" (thorad).

I-verb gerunds are nice and easy too. Add (-ed) to the root.
Example: From the root "halt" (dar-) we get "halting" (dared).

Gerunds take the place of infinitives in Sindarin grammar and are used as nouns. Watch the accusative pronoun carefully. When the gerund is happening to an accusative pronoun, the pronoun precedes it like it was any other verb. It does not go before the active verb. Be careful with the gerund, sometimes in English we leave the "to" out of infinitives. To make certain, add (-ing) onto the verb. If the sentence still makes sense, then it is a hidden infinitive and you need the gerund.
For example:
The orcs want me to kill you. - In yrch nin anírar le daged.
I heard someone speak to him. - Lastannen ben peded athe.

When the gerund is being a noun, it can't be made plural, but otherwise it acts as though it is a normal noun and undergoes lenition.
Make the killing stop. - Caro i naged nuitha.

Negation

Negation is done through prefixes, after which follows Soft Lenition.

First, there is the verb negation, "don't" (av[o]-). Before a verb, add (av-). If the verb is imperative, place (avo) before it.
For example:
I didn't halt. - Av-dhernin.
Don't halt! - Avo dharo!

Then there is noun negation, "no/none" (ú-). This prefix is added to nouns, but when the noun is the direct object of a verb, it is added to the verb. This noun never is made plural. This also applies to gerunds acting as nouns.
For example:
I didn't fence any gardens. (I fenced no gardens) - Ú-thorannen hant.
He fenced nothing. - Ú-thorant nad.
No one halted them. - Ú-ben di darn.
We halted none. - Ú-dhernim.

Finally there is adjective negation, "not/un-/non-" (al-). This prefix is only added to adjectives.
For example:
The unfenced garden halted no animals. - I hant al-thorannen ú-dharn lavan.
That is not yours. - Ta al-gîn.

Homework

Symbols that you may need:
áéíóúý
ÁÉÍÓÚÝ
âêîôûŷ
ÂÊÎÔÛŶ

Wordbank

Verbs:
Gannada- (play a harp)
Mad- (eat)
Nautha- (conceive a thought, think)
Rib- (rush, fly)

Nouns:
Aes (food)
Aur (day)
Caras (city)
Lain (thread)
Lebed (finger)
Lim (fish)

Adjectives:
Lhain (skinny)
Laeb (fresh)
Lhaew (ill)
Puig (clean)

Conjugate the verbs in the word bank into their infinitive and gerund forms.

Gannada - gannado / gannadad
Mad - medi / maded
Nautha - nautho / nauthad
Rib - ribi / ribed

Conjugate the verbs in the word bank into present tense, third person singular, with a negative prefix.

Gannada - av-ngannada
Mad - av-vâd
Nautha - av-nautha
Rib - av-rîb

Conjugate the verbs in the word bank into the imperative tense with a negative prefix.

Gannada - avo ngannado
Mad - avo vado
Nautha - avo nautho
Rib - avo ribo

Put negative prefixes onto all of the verbs' gerunds, the nouns, and the adjectives in the word bank.

Gannada - ú-ngannadad
Mad - ú-vaded
Nautha - ú-nauthad
Rib - ú-ribed
Aes - ú-aes
Aur - ú-aur
Caras - ú-garas
Lain - ú-lain
Lebed - ú-lebed
Lim - ú-lim
Lhain - al-thlain
Laeb - al-laeb
Lhaew - al-thlaew
Puig - al-buig

Translate the following sentences into Sindarin.

[informal] If you don't think while you play the harp, your fingers will fly on the strings. - Ae av-nauthach ir gannadach lebid ribithar erin lain.
I didn't eat a spoiled fish, and I am not ill. - Av-mennin lim al-laeb a ni al-thlaew.
We thought he didn't play the harp. - Nauthannem i e av-ngannant.
They are skinny; they haven't eaten any food for days. - Di lhain, ú-vennir aes ned oer.

Translate the following sentences into English.

Gannadad mado aes lîn. - To play a harp, eat your food.
E rimp anin charas al-buig. - He rushed to the unclean city.
Nauthon i maded al-thlaew. - I think that eating is not ill.